wl1271: Fix PSM entry

Currently the PSM entry function assumes successful operation, and enables
ELP, BET and beacon filtering right away. This is bad, because the PSM
entry may fail due to environmental issues, which will cause the ELP, BET and
beacon filtering to be illegally enabled (because FW remains in active state.)

Fix this by enabling ELP, BET and beacon filtering only after successful entry,
and by ensuring the firmware is in active mode after the failure.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>